1. Field of the Invention
The present invention relates to a wind turbine blade.
2. Description of the Related Art
There is an increasing trend for longer and longer wind turbine blades. This has led to a recent development whereby each blade is manufactured in a number of sections which are connected end-to-end in order to form the finished blade. The joint between these adjacent sections is critical to the operation of the blade.
Such wind turbine blades generally comprise an aerodynamic fairing supported by a longitudinally extending spar. In any blade consisting of a number of segments as referred to above, the joint between adjacent spar sections is critical as it is through this that the vast majority of the load is transmitted.
US 2008/0069699 discloses a wind turbine blade which is manufactured in two sections which are subsequently joined. In the region adjacent to the joint, the spar is provided with enlarged portion which has a number of teeth which mesh with corresponding teeth on the adjacent spar section. The surface shell structure can also be provided with reinforcing strips which are similarly provided with inter-meshing teeth and intermediate connection means of various constructions.
WO 2009/034291 discloses joints between adjacent spar caps in which the width of the spar caps tapers adjacent to the joint. The tapered surfaces are then bonded together and additional supports are provided between the shear webs to strengthen the joint.
Another example of adjoining adjacent spar sections is disclosed in US 2009/0162208. The joint between spar segments disclosed in this document is shown in FIG. 1.
This comprises a first spar segment 1 and a second spar segment 2. Each spar segment consists of a shear web 3 with a spar cap 4 on each side. In the first spar segment 1, the shear web 3 stops short of the end of the spar caps. Each of the spar caps has a tapered end with an inclined inwardly facing tapered lower surface.
For the second spar segment 2, the shear web and spar caps are coterminous. In this case, the outer faces of the spar caps taper inwardly to provide a tapered surface 6 complementary to the respective tapered surface 5 on the first spar cap 1. The two spar caps are then bonded together between the adjacent tapered surfaces 5, 6 to form the completed spar. By tapering the depth of the spar caps, rather than the width as in WO 2009/034291, the bond area is greatly increased thereby increasing the strength of the joint.
However, this design suffers from a number of drawbacks. Firstly, as will be apparent from FIG. 1, in the first spar segment 1, the spar caps protrude beyond the edge of the shear web and are long, thin components which are unsupported, at this point, by the shear web 3. These portions of the spar cap can extend for several metres. As such, they are highly vulnerable to damage in transit and assembly.
Secondly, the two spar sections themselves are long components extending for up to 50 metres (but are more typically 20-30 metres). It is difficult, in practice, to provide stable support for the two segments as the adhesive is applied to the inclined surfaces and then to move one with respect to the other into the correct alignment and then hold it in position until the laminating process is completed and the resin has cured.
Finally, as the direction in which the inclined surfaces 5, 6 extend is close to the direction in which the two components are brought together in view of the shallowness of the taper, this can cause problems with the adhesive during assembly. In particular, any misalignment on assembly can scrape the adhesive from the surfaces 5, 6 particularly at the leading edge of the spar caps 4 of the second spar segment leading to voids in the bond line which causes problems for the reliability of the joint and ultimately failure of the joint. The spar is then surrounded by an aerodynamic fairing F to complete the blade.